The two starting materials, included in formula (II), are 1,1,1-trichloro-2,2,2-trifluoroethane (CF.sub.3 CCl.sub.13) and 1,1-dichloro-1,2,2,2-tetrafluoroethane (CF.sub.3 CFCl.sub.2), in which the substitution of a chlorine atom by a hydrogen atom leads, respectively, to 1,1-dichloro-2,2,2-trifluoroethane (CF.sub.3 CHCl.sub.2) and to 1-chloro-1,2,2,2-tetrafluoroethane (CF.sub.3 CHFCl).
The catalytic hydrogenation of the compounds (II) has already been described, but the selectivities for the product corresponding to the removal of a single chlorine atom are low. Thus, the hydrogenolysis of 1,1-dichloro-1,2,2,2-tetrafluoroethane at 280.degree. C. on a catalyst containing 5% of a palladium on charcoal (British Patent No. 1,578,933) yields a product containing 70% of 1,1,1,2-tetrafluoroethane. Similar results are obtained by C. GERVASUTTI, et al., Fluorine Chemistry, 1, 1-20 (1981) on a catalyst containing 0.5% of palladium on charcoal. At 170.degree. C., the hydrogenolysis of 1,1-dichloro-1,2,2,2-tetrafluoroethane leads to 76% of 1,1,1,2-tetrafluoroethane. To solve the problem of the removal of a single chlorine atom, it is necessary to resort, according to Japanese patent application No. 106,051/82 (publication JP 222038/83) to a chemical reduction with the zinc/ethanol system. Under the conditions described, the selectivity of the hydrogenolysis of 1,1,1-trichloro-2,2,2-trifluoroethane to 1,1-dichloro-2,2,2-trifluoroethane reaches 90%. However, this process has the drawback of using costly metallic zinc, and of yielding zinc chloride as a by-product which must be destroyed.
The preceding references are hereby incorporated by reference.